Heretofore, as a signal receiving system to be applied to mobile telephone equipment, superheterodyne system has been widely used. This superheterodyne system is so defined that the frequency of a received RF (Radio Frequency) signal (hereinafter referred to as a received signal frequency) is converted to a different intermediate frequency (IF) for the receiving and processing purpose.
However, when the superheterodyne system is applied to a mobile telephone, it is necessary to provide a circuit element, which is difficult to be IC (Integrated Circuit) chipped, such as a filter for temporarily converting the received signal frequency to the intermediate frequency in a signal receiving processing circuit. Thus, this superheterodyne system shows a tendency that the circuit scale of the signal receiving processing circuit becomes relatively large sized.
Thus, recently it has been proposed to apply the signal receiving system called as a direct conversion system(hereinafter referred to as DCR) to the mobile telephone equipment. This DCR system has an advantage that the signal receiving processing circuit can be configured without any circuit element, such as the filter for the frequency conversion by conducting the signal receiving processing by not changing the received signal frequency of the received RF signal.
Moreover, with the rapid popularization of the mobile telephone equipment in recent years, the number of communication lines that one wireless communication system can keep is becoming insufficient, and multiple kinds of wireless communication systems using different frequency bands have been started.
Thus, in recent years, mobile telephone equipment capable of using multiple kinds of wireless communication systems, such as GSM (Global System for Mobile Communications) adopting a wireless communication system called as TDMA (Time Division Multiple Access) system and using 900 [MHz] band wireless communication frequency, DCS (Digital Cellular System) adopting TDMA system and using 1.8 [GHz] band wireless communication frequency, and UMTS (Universal Mobile Telecommunications system) using 2 [GHz] band wireless communication frequency adopting a wireless communications system called as W-CDMA (Wideband-Code Division Multiple Access) system has been considered (hereinafter this mobile telephone equipment is referred to as a multi-band mobile telephone).
Furthermore, in this multi-band mobile telephone, it is required to provide signal receiving processing circuits corresponding to GSM, DCS and UMTS respectively. Thus, it is considered that applying the DCR system to the multi-band mobile telephone allows this multi-band mobile telephone smaller sized and lighter weighted.
By the way, in a narrow-band digital conversion system used for GSM and DCS, a circuit for suppressing DC offset by a DC feedback cannot be used when the DC offset component generated by DCR is put out to a receiver detection output (i.e., baseband (BB) signal).
The reason is that when such controlling circuit is used, the low band of the frequency is cut out and a part of demodulation signal data is dropped.
FIG. 4 is a Figure for explaining the DC offset generation mechanism in DCR. In the signal receiving processing circuit of DCR, the received RF signal is directly quadrature demodulated using the local oscillation signal of local oscillation frequency (fLO) equal to the received signal frequency (fRF), and the baseband signals I and Q quadrature each other are obtained.
However, at the same time, resulting from the fact that the received signal frequency and the local oscillation frequency are equal, the local oscillation signal is leaked out to the other part of the circuit via the passage shown by a broken line in FIG. 4, and because of non-linear characteristic of each circuit the local oscillation signal is mixed with the received RF signal (this is called as self-mixing). As a result, the DC component is overlapped onto the I/Q outputs of the baseband (BB).
This DC component is unnecessary component for the baseband data, and is considered equivalent to noise.
Moreover, in the leakage passage of the local oscillation signal, there are such passages as a local oscillation signal entering into the input unit of the received RF signal through the other circuit block via a substrate forming the receiving chip, entering into a signal on a print substrate being equipped with the chip, and a local oscillation signal jumping into the antenna via space and mixed up with the received RF signal.
Then, the DC component is generated when the leakage of the local oscillation signal through these multiple leakage passages occurs alone or combined.
Furthermore, when the DC offset component is too large, the offset component is overlapped onto a DC bias voltage of the circuit. As a result, the operation point becomes a power source or a grounding potential, which could be cause of mis-operation.
Because of the above reasons, there are few examples where DCR has been put into a practical use. Even an existing practical usage is only applied to a system in which a demodulation of the DC component is not required, that is the system using the modulation system in which the DC offset compensation by said DC feedback is possible.
Therefore, in the system using the narrow band modulation such as GSM and DCS, it becomes necessary to prevent the local oscillation signal from entering into the RF input.
At this point, regarding the multi-band mobile telephone, the circuit configuration of the signal receiving processing circuit for realizing DCR is explained as follows.
More specifically, as shown in FIG. 5, the signal receiving processing circuit 1 comprises a quadrature demodulation unit 2 for directly quadrature demodulating the received RF signal using the local oscillation frequency equal to the received signal frequency and a frequency generating unit 3 for generating the local oscillation frequency.
Then, this signal receiving processing circuit 1 operates by switching among the GSM mode, the DCS mode, and the UMTS mode according to GSM, DCS, and UMTS. Thus, the received RF signals of GSM, DCS and UMTS are quadrature demodulated respectively.
In the GSM mode, when the received RF signal transmitted from a base station for GSM (not shown in Figures) is received by an antenna element (not shown in Figures), the signal receiving processing circuit 1 feeds that received RF signal into a variable gain low-noise amplifier for GSM 10 in the quadrature demodulation unit 2.
The received RF signal, after being amplified at the variable gain low-noise amplifier 10, is put into a quadrature demodulator 14 comprised of frequency mixers 11, 12 and a polyphase filter 13, and is multiplied by the local oscillation frequency. Here, DCR is realized by equalizing the local oscillation frequency to the received RF signal.
As a result, the I/Q quadrature baseband (BB) signals are obtained from the outputs of the frequency mixers 11 and 12, and said quadrature baseband signals are transmitted to the received-signal baseband processing unit (not shown in Figures) of the later stage.
Furthermore, in the DCS mode, when the received RF signal transmitted from the base station for DCS (not shown in Figures) is received by the antenna element, the signal receiving processing circuit 1 feeds the received RF signal into a variable gain low-noise amplifier for DCS 20.
The received RF signal, after being amplified at the variable gain low-noise amplifier 20, is put into a quadrature demodulator 24 comprised of frequency mixers 21 and 22 and a polyphase filter 23, and then, is multiplied by the local oscillation frequency equal to the received signal frequency.
As a result, I/Q quadrature baseband (BB) signals are obtained from the outputs of the frequency mixers 21 and 22, and are transmitted to the received-signal baseband processing unit (not shown in Figures) of the later stage.
Furthermore, in the UMT mode, when the received RF signal transmitted from a base station for UMTS (not shown in Figures) is received by the antenna element, the signal receiving processing circuit 1 feeds the received RF signal into a variable gain low-noise amplifier for UMTS 30.
The received RF signal, after being amplified at the variable gain low-noise amplifier 30, is put into a quadrature demodulator 34 comprised of frequency mixers 31 and 32 and a polyphase filter 33, and then, is multiplied by the local oscillation frequency equal to the received signal frequency.
As a result, I/Q quadrature baseband (BB) signals are obtained from outputs of the frequency mixers 31 and 32, and is transmitted to the received-signal baseband processing unit (not shown in Figures).
At this point, in the GSM mode, a channel PLL (Phase-Locked Loop) unit 40 outputs a frequency signal of a predetermined oscillation frequency in a voltage controlled oscillator for GSM 41 and feeds it into an output frequency divider 42. Then, the frequency signal, after being frequency divided by the output frequency divider 42, is put into the channel PLL 43 as an output frequency divided signal.
Furthermore, the channel PLL unit 40 captures the standard frequency signal of the standard frequency supplied from a temperature control crystal oscillator 44 into a standard frequency divider 45. The standard frequency signal, after being frequency divided by the standard frequency divider 45, is put into the channel PLL 43 as a standard frequency divided signal.
Then, the channel PLL 43 compares the phase difference between the output frequency divided signal and the standard frequency divided signal, and then outputs the phase difference to a loop filter 46. The loop filter 46 converts the input phase difference component to the DC voltage by integrating the input phase difference component, and applies it onto the control terminal of the voltage controlled oscillator 41.
With this arrangement, the channel PLL unit 40 transmits the frequency signal of the voltage controlled oscillator 41 to a frequency mixer 51 of a mixer unit 50 under the condition where the frequency of the output frequency divided signal is converged toward the frequency of the standard frequency divided signal.
On the other hand, in the GSM mode, a fixed PLL unit 60 outputs the frequency signal of a predetermined oscillation frequency in a voltage controlled oscillator for GSM 61 and feeds it into an output frequency divider 62. The frequency signal, after being frequency divided at the output frequency divider 62, is put into the fixed PLL 63 as an output frequency divided signal.
Furthermore, the fixed PLL unit 60 captures the standard frequency signal of the standard frequency supplied from the temperature control crystal oscillator 44 into the standard frequency divider 64. The standard frequency signal, after being frequency divided at the standard frequency divider 64, is put into the fixed PLL 63 as a standard frequency divided signal.
Then, the fixed PLL 63 compares the phase difference between the output frequency divided signal and the standard frequency divided signal, and outputs the phase difference to the loop filter 65. The loop filter 65, after converting the input phase difference component to the DC voltage by integrating the input phase difference component, and applies it to the control terminal of the voltage controlled oscillator 61.
Thus, the fixed PLL unit 60 transmits the frequency signal of the voltage controlled oscillator 61 to the frequency mixer 51 under the condition in which the frequencies of the output frequency divided signal and the standard frequency divided signal converge to the equal level.
The frequency mixer 51 generates the frequencies of the sum and difference of these two frequencies by mixing these two frequency signals and puts these into a band-pass filter 52.
Here, the frequency formed by the difference of those two frequencies is the local oscillation frequency that is equal to the received signal frequency, and the frequency formed by the sum of the two frequencies is an image frequency that is unnecessary for the demodulation.
Accordingly, the cut-off frequency of the band-pass filter 52 is properly selected, and the band-pass filter 52 selectively passes only the lower local oscillation frequency out of the two frequencies (the local oscillation frequency and the image frequency) and puts it into the polyphase filter 33 of the demodulator 14.
At this point, the mixer unit 50 generates the local oscillation frequency right in front of the input terminal of the polyphase filter 33. Accordingly, the signal receiving processing circuit 1 remarkably shortens the transmission line of the local oscillation frequency and greatly decreases DC offset from the quadrature baseband signal.
In this connection, the band-pass filter 52 eliminates the image frequency as described above. Therefore, the quadrature demodulator 14 demodulates the received RF signal having the received signal frequency equal to the image frequency, and thus prevents the original received RF signal (i.e., the received RF signal having the received signal frequency equal to the local oscillation frequency) from being difficult to be demodulated.
Furthermore, the channel PLL unit 40 feeds, in the DCS mode, frequency signals of the prescribed oscillation frequencies generated at the channel PLL 43 and the voltage controlled oscillator for GSM 47 are fed into the frequency mixer 53 as in the same manner as in the GSM mode.
At this point, the fixed PLL unit 60 feeds the frequency signal of the prescribed oscillation frequency generated at the fixed PLL 63 and the voltage controlled oscillator for DCS 66 into the frequency mixer 53 in the same manner as in the GSM mode.
Thus, the frequency mixer 53 generates two frequencies, the sum of the two frequencies and the difference of the two frequencies by mixing the two frequencies. And then, the band-pass filter 54 selectively passes the frequency having the lower frequency out of the two frequencies (local oscillation frequency equal to the received signal frequency) and feeds it into the polyphase filter 23 in the same manner as in the GSM mode.
Furthermore, in the UMTS mode, it feeds the frequency signal having the prescribed oscillation frequency generated at the channel PLL 43 and the voltage controlled oscillator for UMTS 48 into the frequency mixer 55 in the same manner as in the GSM mode.
At this point, the fixed PLL unit 60 puts the frequency signals having the prescribed oscillation frequencies generated at the fixed PLL 63 and the voltage controlled oscillator for UMTS 67 into the frequency mixer 55 in the same manner as in the GSM mode.
Thus, the frequency mixer 55, by mixing these two frequencies, generates two frequencies, the sum and the difference of the two frequencies. Then, the band-pass filter 56 selectively passes only the frequency having the lower frequency (local oscillation frequency equal to the received signal frequency) out of these two frequencies by the band-pass filter 56 being provided near to the demodulator 34, and feeds it into the polyphase filter 33.
Thus, the signal receiving processing circuit 1 can greatly decrease the DC offset from the received baseband signals in the GSM mode, the DCS mode, and the UMTS mode respectively.
By the way, in the signal receiving processing circuit 1 thus configured, as it is apparent from FIG. 5, voltage controlled oscillators for GSM, DCS and UMTS respectively, 41, 47, 48 and 61, 66, 67 are needed in the channel PLL unit 40 and the fixed PLL unit 60.
Furthermore, it is necessary to have frequency mixers for GSM, DCS and UMTS, 51, 53, 55 and band-pass filters for GSM, DCS and UMTS 52, 54, 56 in the mixer unit 50. Thus, it is obvious that the circuit scale of the signal receiving and processing circuit 1 becomes larger due to such configuration.
Furthermore, since this signal receiving processing circuit 1 can only be formed by the band-pass filters 52, 54 and 56 which are formed of discrete components, which are difficult to be integrated onto the IC, the circuit scale further increases. As a result, although DCR is applied in the signal receiving processing circuit 1, there is a problem still unsolved for making the circuit scale smaller.
Therefore, as shown in FIG. 6, where corresponding parts of FIG. 5 are given the same reference numerals, it is considered that an image removal mixer that can be formed by integrating in an IC chip is provided in place of said mixer unit 50 (FIG. 5) in the frequency generating unit 71 of the signal receiving processing circuit 70.
In this case, the image removal mixer is a circuit capable of taking out only such as the difference frequency out of the sum and difference frequencies when the two frequencies are mixed.
More specifically, in the GSM mode, this circuit feeds the frequency signal generated at the channel PLL 40 and the voltage controlled oscillator for GSM 41 into the polyphase filter 73 of the image removal mixer 72. Said frequency signal is split into two signals in quadrature in the polyphase filter 73 and fed into one side input terminals of the frequency mixers 74 and 75 respectively.
At this point, the frequency signal generated at the fixed PLL 63 and the voltage controlled oscillator for GSM 61 is fed into the frequency divider 76 of the image removal mixer for GSM 72. And after being frequency divided in half at the frequency divider 76, the frequency signal is converted to two signals in quadrature and these two signals are fed into the other side input terminals of the frequency mixers 74 and 75.
At this point, when the phase relation of the four signals to be put into the frequency mixers 74 and 75 is properly selected, the image frequency is removed by adding output signals of these frequency mixers 74 and 75 at the adder 77 of the later stage and only the local oscillation frequency equal to the received signal frequency can be obtained.
Then, the local oscillation signal having the local oscillation frequency is fed into the polyphase filter 13 from the adder 77. Thus, in the GSM mode, the received RF signals are quadrature demodulated directly in the quadrature demodulator for GSM 14.
Furthermore, in the DCS mode, the frequency signal generated at the channel PLL 40 and the voltage controlled oscillator for DCS 47 is divided into two signals in quadrature at the polyphase filter 81 in the image removal mixer for DCS 80, and these are fed respectively into frequency mixers 82 and 83.
At this point, the frequency signal generated at the fixed PLL 63 and the voltage controlled oscillator for DCS 66 is frequency divided in half via the frequency divider 84, and converted to two signals in quadrature and these signals are fed respectively into the frequency mixers 82 and 83.
At this point, the phase relation of the four signals to be put into the frequency mixers 82 and 83 is selected properly in advance, and the output signals of these frequency mixers 82 and 83 are added at the adder 85. As a result, only the local oscillation frequencies equal to the received signal frequency are generated and fed into the polyphase filter 23.
Furthermore, in the UMTS mode, the frequency signal generated at the channel PLL 40 and the voltage controlled oscillator for UMTS 48 is divided into two signals in quadrature by the polyphase filter 91 in the image removal mixer for UMTS 90 in the same manner as in the GSM mode and in the DCS mode, and these are fed respectively into the frequency mixers 92 and 93.
At this time the frequency signal generated at the fixed PLL 63 and the voltage controlled oscillator for DCS 67 is frequency divided in half via the frequency divider 94, and converted to two signals in quadrature and these are fed into frequency mixers 92 and 93 respectively.
At this point, the phase relation of the four signals to be put into frequency mixers 92 and 93 is properly selected, and the output signals of these frequency mixers 92 and 93 are added by the adder 95. As a result, only the local oscillation frequencies equal to the received signal frequency are generated, and these are fed into the polyphase filter 33.
With this arrangement, the signal receiving processing circuit 70 can generate the desired local oscillation signal not using the band-pass filter that is difficult to be integrated in an IC chip.
However, as is clear from FIG. 6, since the image removal mixers for GSM, DCS and UMTS, 72, 80 and 90 are provided in the signal receiving processing circuit 70, the circuit scale in the IC increases. As a result, this signal receiving processing circuit 70 is still insufficient to simplify the circuit configuration.